Computer keyboards are often supplemented with other types of input devices. Some of these supplemental input devices, such as a trackball or mouse, include components that are moved by the user. Encoder mechanisms detect motion of the movable components and provide to the computer digital information representative of movement of the trackball or mouse. This information is typically used by the computer for controlling the movement of a cursor or pointer on a computer display screen.
A mouse is connected to a computer by a flexible cable. The mouse houses a ball that is rolled along a surface adjacent to the computer. Use of a mouse with portable, compact, "notebook" type computers is inconvenient because the mouse must be transported as a separate component. The user must attach the mouse each time the computer is used and detach it when finished.
Some input devices, such as trackballs, may be built into portable computers. Built-in devices have been mounted in the vicinity of the computer keyboard, with the movable component projecting from the upper surface of the keyboard. Devices so mounted to the keyboard necessarily require keyboard surface space, a requirement that is generally contrary to design goals that seek to minimize the keyboard size to make the computer as compact as possible.